1. Field of the Invention
A method and system for determining the electrical resistance of a field-effect transistor (FET).
2. Description of the Prior Art
A field-effect transistor (FET) is often used as a switch in power electronics to control current to larger electrical load, such as an electrical motor. As electrical current (I) flows through the FET, a drain-to-source voltage (VDS) is created by virtue of the FET on resistance (RDS). It is useful to know the current and power (P) so that the current provided to the load can be directly and accurately controlled and to prevent exceeding the operating temperature of the FET.
The drain-to-source voltage (VDS) of an FET can be measured directly with well known electrical devices. It is difficult to determine the real time current and power of an FET because the resistance (RDS) of the FET can vary significantly with temperature. In some instances, the resistance (RDS) of the FET can be twice as high at 175° C. as at 25° C. If the temperature at the junction (TJ) is known, the resistance (RDS) of the FET can be determined by referring to the characteristics of the FET as provided in the data sheets of the manufacturer. However, it is difficult to measure the temperature at the junction (TJ) of the FET directly, albeit it is possible to use a thermistor placed close to the FET to measure the temperature (Tth).